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Same-sex marriage is thought to have majority support but there are also groups sternly against it

"We encourage every Australian to vote in this survey, to have their say," Mr Turnbull said last week.

The postal survey has drawn controversy over its A$122m (£75m, $97m) cost, fears that it will prompt hate-filled campaigns, and the fact that voting is voluntary. Australian has compulsory voting for elections.

It has also been criticised by same-sex marriage advocates as an unnecessary delay in resolving the debate, which they say could be settled immediately in parliament.